U.S. patent application number 12/007516 was filed with the patent office on 2009-07-16 for overload control method for a wireless cellular network.
Invention is credited to Rainer W. Bachl, Fang-Chen Cheng, Jung Ah Lee.
Application Number | 20090179755 12/007516 |
Document ID | / |
Family ID | 40626682 |
Filed Date | 2009-07-16 |
United States Patent
Application |
20090179755 |
Kind Code |
A1 |
Bachl; Rainer W. ; et
al. |
July 16, 2009 |
Overload control method for a wireless cellular network
Abstract
A method may include determining a metric for at least one
physical resource block of a wireless cellular network in at least
a one cell. Each physical resource block may include a set of
frequencies, and/or the metric may be based on interference on the
at least one physical resource block in the at least one cell. A
determination of whether the metric violates a metric threshold may
be made, and an overload indicator may be sent to at least one
other cell if the metric violates the metric threshold.
Inventors: |
Bachl; Rainer W.;
(Nurembert, DE) ; Cheng; Fang-Chen; (Randolf,
NY) ; Lee; Jung Ah; (Pittstown, NJ) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 8910
RESTON
VA
20195
US
|
Family ID: |
40626682 |
Appl. No.: |
12/007516 |
Filed: |
January 11, 2008 |
Current U.S.
Class: |
340/540 ;
455/405 |
Current CPC
Class: |
H04W 72/082 20130101;
H04W 72/0426 20130101; H04W 92/20 20130101 |
Class at
Publication: |
340/540 ;
455/405 |
International
Class: |
G08B 21/00 20060101
G08B021/00; H04M 11/00 20060101 H04M011/00 |
Claims
1. A method comprising: determining a metric for at least one
physical resource block of a wireless cellular network in at least
one cell, each physical resource block including a set of
frequencies, the metric based on interference on the at least one
physical resource block in the at least one cell; determining if
the metric violates a metric threshold; and sending an overload
indicator to at least one other cell if the metric violates the
metric threshold.
2. The method of claim 1, wherein the metric is a ratio of power
spectral density of interference on the at least one physical
resource block in the at least one cell relative to a maximum
allowed transmit power of one or more users in the at least one
cell.
3. The method of claim 2, wherein the metric is calculated in
accordance with the following equation: M k =
Normalized_Interference _PSD = k Interference ( PRB k ) / N P Max
##EQU00002## wherein the numerator is an average interference for
each physical resource block of the at least one physical resource
block, N is a number of the at least one physical resource block,
and P.sub.max is the maximum allowed user transmit power of the one
or more users in the at least one cell.
4. The method of claim 1, wherein the metric is a ratio of power
spectral density of interference on the at least one physical
resource block in the at least one cell relative to a maximum
allowed transmit power spectral density of one or more users in the
at least one cell.
5. The method of claim 1, wherein the sending the overload
indicator to the at least one other cell includes selecting the at
least one other cell from a neighbor cell list of the at least one
cell.
6. The method of claim 1, wherein the at least one physical
resource block corresponds to a low interference frequency zone of
the at least one cell, the low interference frequency zone
including a set of frequencies having a target power spectral
density in the at least one cell which constrains a power spectral
density of interference from neighboring cells.
7. The method of claim 1, wherein the overload indicator is one
bit.
8. The method of claim 1, wherein the sending the overload
indicator includes sending the overload indicator via an interface
between the at least one cell and the at least one other cell.
9. A method comprising: receiving an overload indicator from at
least one cell in at least one other cell, the overload indicator
indicating that a metric based on interference on at least one
physical resource block of a wireless cellular network in the at
least one cell violates a metric threshold, each physical resource
block including a set of frequencies.
10. The method of claim 9, wherein the metric is a ratio of power
spectral density of interference on the at least one physical
resource block in the at least one cell relative to a maximum
allowed transmit power of one or more users in the at least one
cell.
11. The method of claim 10, wherein the metric is calculated in
accordance with the following equation: M k =
Normalized_Interference _PSD = k Interference ( PRB k ) / N P Max
##EQU00003## wherein the numerator is an average interference for
each physical resource block of the at least one physical resource
block, N is a number of the at least one physical resource block,
and P.sub.max is the maximum allowed user transmit power of the one
or more users in the at least one cell.
12. The method of claim 9, wherein the metric is a ratio of power
spectral density of interference on the at least one physical
resource block in the at least one cell relative to a maximum
allowed transmit power spectral density of one or more users in the
at least one cell.
13. The method of claim 9, further comprising: identifying, at the
at least one other cell, a user producing a strongest interference
on the at least one physical resource block in the at least one
cell in response to the overload indicator; and at least one of
reducing power spectral density of a target transmit power of the
identified user and modifying a frequency allocation of the
identified user.
14. The method of claim 13, wherein the reducing the power spectral
density of the target transmit power of the identified user reduces
the power spectral density of the target transmit power of the
identified user by a step size.
15. The method of claim 13, wherein the step size is a step size
pre-configured at the at least one other cell.
16. The method of claim 13, wherein the modifying the frequency
allocation of the identified user includes assigning a different
set of frequencies to the identified user.
17. The method of claim 13, wherein the identifying the user
producing the strongest interference is based on path loss
reporting from the user to the at least one other cell of the user
and to a strongest neighbor cell of the at least one other cell of
the user.
18. The method of claim 13, wherein the identifying the user
producing the strongest interference is based on power headroom
reporting and a path loss difference between the at least one other
cell of the user and a strongest neighbor cell of the at least one
other cell of the user.
19. The method of claim 9, wherein the overload indicator is one
bit.
20. A wireless cellular network comprising: at least one cell
configured to determine a metric for at least one physical resource
block in the at least one cell, each physical resource block
including a set of frequencies, the metric based on interference on
the at least one physical resource block in the at least one cell,
wherein the at least one cell is configured to determine if the
metric violates a metric threshold and send an overload indicator
to at least one other cell if the metric violates the metric
threshold.
Description
BACKGROUND
[0001] 1. Field
[0002] The invention is related to an overload control method.
[0003] 2. Description of Related Art
[0004] Conventional overload control schemes are used for
inter-cell power control. A conventional overload control scheme
may complement an Inter-cell Interference Coordination (ICIC)
scheme. The conventional overload control scheme detects an
interference overload event at each cell and sends an overload
indicator to neighboring cells.
SUMMARY
[0005] According to an example embodiment, a method may include
determining a metric for at least one physical resource block of a
wireless cellular network in at least a one cell. Each physical
resource block may include a set of frequencies, and/or the metric
may be based on interference on the at least one physical resource
block in the at least one cell. A determination of whether the
metric violates a metric threshold may be made, and an overload
indicator may be sent to at least one other cell if the metric
violates the metric threshold.
[0006] According to another example embodiment, a method may
include receiving an overload indicator from at least one cell in
at least one other cell, the overload indicator indicating that a
metric based on interference on at least one physical resource
block of a wireless cellular network in the at least one cell
violates a metric threshold. Each physical resource block may
include a set of frequencies.
[0007] According to still another example embodiment, a wireless
cellular network may include at least one cell. The at least one
cell may be configured to determine a metric for at least one
physical resource block in the at least one cell. Each physical
resource block may include a set of frequencies. The metric may be
based on interference on the at least one physical resource block
in the at least one cell. The at least one cell may be configured
to determine if the metric is greater than a metric threshold
and/or send an overload indicator to at least one other cell if the
metric is greater than the metric threshold.
[0008] According to an example embodiment, a wireless cellular
network may include at least one cell and at least one other cell.
The at least on other cell may be configured to receive an overload
indicator from the at least one cell. The overload indicator may
indicate that a metric based on interference on at least one
physical resource block in the at least one cell violates a metric
threshold. Each physical resource block may include a set of
frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Example embodiments will become more fully understood from
the detailed description given herein below and the accompanying
drawings, wherein like elements are represented by like reference
numerals, which are given by way of illustration only and thus are
not limiting of the scope and wherein:
[0010] FIG. 1 illustrates a wireless cellular network according to
an example embodiment;
[0011] FIG. 2 illustrates an example semi-static Inter-Cell
Interference Coordination (ICIC) scheme according to an example
embodiment; and
[0012] FIG. 3 is a signal flow and process diagram illustrating an
overload control method according to an example embodiment.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0013] In the following description, for purposes of explanation
and not limitation, specific details are set forth such as
particular architectures, interfaces, techniques, etc., in order to
provide a thorough understanding of example embodiments. However,
it will be apparent to those skilled in the art that example
embodiments may be practiced in other illustrative embodiments that
depart from these specific details. In some instances, detailed
descriptions of well-known devices, circuits, and methods are
omitted so as not to obscure the description of example embodiments
with unnecessary detail. All principles, aspects, and embodiments,
as well as specific examples thereof, are intended to encompass
both structural and functional equivalents thereof. Additionally,
it is intended that such equivalents include both currently known
equivalents as well as equivalents developed in the future.
[0014] Example embodiments are discussed herein as being
implemented in a suitable computing environment. Although not
required, example embodiments will be described in the general
context of computer-executable instructions, such as program
modules or functional processes, being executed by one or more
computer processors or CPUs. Generally, program modules or
functional processes include routines, programs, objects,
components, data structures, etc. that perform particular tasks or
implement particular abstract data types. The program modules and
functional processes discussed herein may be implemented using
existing hardware in existing communication networks. For example,
program modules and functional processes discussed herein may be
implemented using existing hardware at existing radio network
control nodes.
[0015] In the following description, illustrative embodiments will
be described with reference to acts and symbolic representations of
operations (e.g., in the form of signal flow diagrams) that are
performed by one or more processors, unless indicated otherwise. As
such, it will be understood that such acts and operations, which
are at times referred to as being computer-executed, include the
manipulation by the processor of electrical signals representing
data in a structured form. This manipulation transforms the data or
maintains it at locations in the memory system of the computer,
which reconfigures or otherwise alters the operation of the
computer in a manner well understood by those skilled in the
art.
[0016] It will be understood that, although the terms first,
second, third, etc. may be used herein to describe various
elements, components, regions, layers and/or sections, these
elements, components, regions, layers and/or sections should not be
limited by these terms. These terms are only used to distinguish
one element, component, region, layer or section from another
element, component, region, layer or section. Thus, a first
element, component, region, layer or section discussed below could
be termed a second element, component, region, layer or section
without departing from the teachings of the example
embodiments.
[0017] The terminology used herein is for the purpose of describing
particular example embodiments only and is not intended to be
limiting. As used herein, the singular forms "a," "an" and "the"
are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, and/or components.
[0018] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which example
embodiments belong. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and will not be
interpreted in an idealized or overly formal sense unless expressly
so defined herein.
[0019] As used herein, the term "mobile" may be considered
synonymous to, and may hereafter be occasionally referred to, as a
mobile unit, mobile station, mobile user, access terminal (AT),
user equipment (UE), subscriber, user, remote station, access
terminal, receiver, etc., and may describe a remote user of
wireless resources in a wireless communication network. The term
"base station" may be considered synonymous to and/or referred to
as a base transceiver station (BTS), base station, NodeB, etc. and
may describe equipment that provides data and/or voice connectivity
between a network and one or more users.
[0020] As is well-known in the art, each of a mobile and a base
station may have transmission and reception capabilities.
Transmission from the base station to the mobile is referred to as
downlink or forward link communication. Transmission from the
mobile to the base station is referred to as uplink or reverse link
communication.
[0021] Reference will now be made to example embodiments, which are
illustrated in the accompanying drawings, wherein like reference
numerals refer to the like components throughout.
Wireless Communication Network
[0022] FIG. 1 illustrates a wireless cellular network 100. The
wireless cellular network 100 may include a plurality of cells 10.
Each of the cells 10 may include a base transceiver station 20
and/or one or more user equipment 30. On skilled in the art will
readily recognize that cells, base transceiver stations, and user
equipment described below in relation to example embodiments may be
any of the cells 10, base transceiver stations 20, and user
equipment 30 illustrated in FIG. 1
[0023] Overload control methods may complement Inter-Cell
Interference Coordination (ICIC) schemes. In a semi-static ICIC
scheme, multiple frequency zones may be configured depending on an
interference target for each zone. A frequency zone may be a group
of selected frequencies, e.g., a group of channels. A semi-static
ICIC scheme may create low interference zones having lower
interference than other zones, for example, by constraining power
spectral density (PSD) of interference on the low interference
zones.
Semi-Static ICIC Scheme
[0024] FIG. 2 illustrates a semi-static ICIC scheme including low
interference zones F1, F2, and F3 for cells i, j, and k,
respectively. Each of the low interference zones may have a target
interference PSD level. User equipment at an edge of neighboring
cells may have constraints on an upper limit of PSD for the low
interference zone F1. The low interference zone F1 may allow the
user equipment at an edge of cell i to be scheduled with relatively
higher PSD because the edge of cell i has a relatively lower
inference level and/or the edge of cell i is not a low interference
zone for any neighboring cells.
[0025] The semi-static ICIC scheme may create favorable sub-bands
of frequencies, which have a lower interference level and allow
edge users to transmit with higher PSD. Conventionally, a
semi-static ICIC scheme relies on downlink path loss and path loss
difference to the user equipment between a serving cell, e.g., the
source cell of the user, and a neighbor cell.
[0026] An overload control method according to an example
embodiment may detect an interference overload event at each cell
relative to the target interference level of the cell and send an
overload indicator OI, e.g., via the well-known X2 interface, to
neighboring cells. The overload indicator may be a one bit
indicator for each physical resource block (PRB) or each group of
PRBs. A PRB is a set of sub-carriers, i.e., frequencies, which may
be allocated to a user at the same time. A PRB may be a set of
consecutive frequencies. A group of PRBs may be a group of
consecutive PRBs or a group of nonconsecutive, e.g., isolated,
PRBs. Accordingly, the overload indicator OI may ensure that edge
user coverage is fulfilled in the presence of interference and/or
reduce any failures in the semi-static ICIC scheme because of
measurement errors (e.g., estimation error in downlink path loss or
path loss difference), mismatch between uplink and downlink path
loss, or an overshoot of an interference level above a target
interference level.
Overload Control
[0027] FIG. 3 is a signal flow and process diagram illustrating an
overload control method according to an example embodiment. As
shown in FIG. 3, a source cell may measure a metric M.sub.k for
uplink interference for each physical resource block PRB or for
each group of PRBs (S1). For example, a metric M.sub.k may be
measured at a lower layer L1, e.g., a physical layer, of the
wireless cellular network 100. A determination of the metric
M.sub.k will discussed in detail below. The source cell may
estimate the metric M.sub.k for each PRB or each group of PRBs and
determine if the metric M.sub.k violates a metric threshold
M.sub.thresh (S2). For example, a determination of whether the
metric M.sub.k violates the metric threshold M.sub.thresh may be
made at higher layers L2/L3, e.g., a resource management layer, of
the wireless cellular network 100. For example, if the metric
M.sub.k exceeds the metric threshold M.sub.thresh an overload
indication event may occur. The source cell may identify one or
more target cells (S3) and send the overload indicator OI, e.g.,
via an X2 interface, to the one or more target cells (S4). The
target cell or cells may be determined by a neighbor cell list,
e.g., a neighbor cell list of the source cell.
[0028] Each of the target cell(s) may identify user equipment
having a strongest interference on the source cell that sent the
overload indicator OI. For example, each cell may identify a
different user equipment. For example, path loss reported from
interfering user equipment to a serving cell (e.g., the source cell
of the user), and the strongest neighbor cell may be used by the
target cell to determine an interference caused by the interfering
user equipment on the source cell. For example, path loss reporting
in medium access control (MAC) protocol data units (PDUs) as part
of scheduling information may be used for the determination.
Alternatively, power headroom reporting and a path loss difference
between the serving cell and the strongest neighbor cell may be
used for the determination. Downlink reference signal transmit
power of the serving cell and the strongest neighbor cell may be
used for the determination.
[0029] The target cell(s), e.g., a target cell scheduler of the
target cell(s), may reduce, e.g., by a desired, or alternatively, a
predetermined step size, a target transmit PSD of the identified
user equipment having the strongest interference for the
corresponding overloaded PRB or PRB group (S5). Alternatively, the
target cell(s) may modify a frequency allocation of the identified
user equipment (S5). Target transmit PSD may be equivalent to a
desired, or alternatively, a predetermined normalized PSD
threshold. Target transmit PSD may be configured for each PRB for
each cell. For example, a target cell power control function may
reduce a power control parameter for the identified user equipment
having the strongest interference by a desired, or alternatively, a
predetermined step size. However, example embodiments are not
limited thereto and the target cell(s) may reduce the power control
parameter by one step or the overload indicator may include more
than one bit to indicate a step size.
[0030] However, the target cell may instead or additionally change
a frequency allocation of the identified user equipment; for
example, if changing the target transmit PSD of the identified user
equipment is not sufficient to reduce the interference level of the
source cell. For example, the target cell may change a frequency
zone of the identified user equipment to a different frequency
zone.
[0031] The target cell(s) need not send a response to the source
cell to inform the source cell about any action taken in response
to the overload indicator OI. However, example embodiments are not
limited thereto and the target cell may send a response to the
source cell in response actions taken based on the overload
indicator OI.
Determining Metric M.sub.k
[0032] The metric M.sub.k may be based on an interference level on
the source cell. For example, the metric M.sub.k may represent
interference PSD, i.e., an interference per PRB. Interference PSD
may be computed as a total interference power over a group of PRBs
divided by the number of PRBs in the group. Interference PSD may
provide a measure with a smaller dynamic range over a wider range
of PRB sizes.
[0033] Absolute interference and Interference over Thermal (IoT)
may be used as metrics for interference level. However, for
absolute interference, if power control is active, power control
may compensate for the increased interference level, thereby
overcoming a rise in interference. IoT may require estimation of
thermal noise power experienced on an uplink. Accordingly, IoT may
create gaps for thermal noise measurement if based on an algorithm
with relatively reasonable complexity. However, creating
measurement gaps may be relatively difficult in asynchronous
network deployment.
[0034] The metric M.sub.k for an overload control method according
to an example embodiment may be based on a relative metric defined
by a ratio of interference PSD relative to a maximum allowed user
equipment transmit power in the source cell. The metric M.sub.k may
be normalized because cell coverage may depend on two factors:
allowed maximum transmit power and the interference PSD. The
allowed maximum user equipment transmit power may depend on cell
implementation and/or may be a function of maximum user equipment
transmit power depending on user equipment power class, coverage
planning, and/or a semi-static ICIC scheme.
[0035] The metric M.sub.k for interference may be defined in
accordance with the following equation:
M k = Normalized_Interference _PSD = k Interference ( PRB k ) / N P
Max ( 1 ) ##EQU00001##
where the numerator of equation (1) is an average interference per
PRB over the PRBs in the group of PRBs, N being the number of PRBs
in the group. The group of PRBs may be a low interference zone
configured by a semi-static ICIC scheme for a source cell.
P.sub.max is the allowed maximum user equipment transmit power.
However, example embodiments are not limited thereto, and
alternative normalization factors, e.g., allowed maximum transmit
PSD, which is related to the power control parameter, may be
used.
[0036] The overload indication event may be triggered, e.g., the
source cell may send the overload indicator OI, if the metric
M.sub.k is greater than the metric threshold M.sub.thresh. The
metric threshold M.sub.thresh may be a parameter separately
configurable for each cell. For example, if the normalized
interference PSD is greater than a normalized interference PSD
threshold, the overload indication event may be triggered. The
parameters for the overload indication event, for example, the
metric M.sub.k, the metric threshold M.sub.thresh, the step size
configuration, and/or the frequency zones, may be operation and
maintenance (OAM) parameters.
[0037] Accordingly, example embodiments may provide an overload
control method including sending an overload indicator if a metric
for interference for a PRB or a group of PRBs is greater than a
threshold for the metric.
[0038] Example embodiments being thus described, it will be obvious
that the same may be varied in many ways. Such variations are not
to be regarded as a departure from the example embodiments, and all
such modifications are intended to be included within the
scope.
* * * * *